Reddit mentions of Orion ShortTube 80 Refractor Telescope Optical Tube Assembly (White)

Album of processing steps, updated as I add to the tutorial

Hello to both new /r/astrophotography visitors and old pros. In this tutorial, I wanted to run through the various steps and options for astrophotography processing in PixInsight using an M8 data set from /u/RupeshJoy852. His hardware is fairly modest: a DSLR and an Orion 80mm ShortTube refractor. He took a set of 90 second exposures at ISO 1600 and then gave the data set to me to see what could be done in PixInsight.

For each step, I'm going to discuss the Newbie, Intermediate, and Experienced options, and I want to show that even with a fairly basic workflow using a PixInsight trial license, you can get decent results.

First step for all three is to get a look at the data.

• Newbie: Use the Blink process to load up all your images and flip through them to see which had good tracking and which have bad streaks. When using a DSLR, the image is often completely dominated by one color, in this case red, so you click the red, green, and blue striped button to be able to unlock the color ratios and really see your data.
  
  
• Intermediate: Same thing, look through your stack of data and select out the good from the rest. You can use the Move button in Blink to select the ones that are good and put them in a separate folder. You should also be shooting darks, flats, and bias frames, so look at those too.
  
  
• Experienced: Use Script>Batch>SubFrame Selector as well to really measure and filter your best frames. You can sort by SNR, FWHM, Eccentricity, and other measurements of your stars.
  
  For DSLR data, you Batch Debayer your data
  
  N: Load Script>Batch Processing>Batch DeBayer and load in your good images. Run it and get the resulting FITS files.
  
  I: You should be shooting darks, flats, and bias frames, so use Batch Preprocessing instead to load all your data in and calibrate. Debayering is part of the process.
  
  *E: Following the advice of /u/EorEquis, you should be Dithering your exposures, so you should be able to skip dark frames -- they might be injecting noise into your final frames! There's an active debate about it, and I still use darks, but I understand the logic.
  
  Align all the images so that you can stack them
  
  
• Load the Process called StarAlignment and load in your newly Debayered FITS files. Pick one of your best frames and use it on top as a reference that all the others will be aligned to. Choose an output directory for all the new aligned images. The defaults should work fine.
  
  
• Same for intermediate and experienced, but if you used Batch Preprocessing above, you will get the aligned frames as output.
  
  "Stack" your images, getting an averaged result with greatly increased signal to noise ratio. We shoot multiple frames so that the uncertainty (noise) in any given frame cancels out with the rest of the frames when averaged. The real astronomical light, the signal, will be revealed, showing the Deep Sky Object.
  
  
• Use Image Integration, load your frames, and then adjust the Pixel Rejection settings. Choose Winsorized Sigma Clipping if you have more than 8 images and try using the defaults. The result is a master light frame that we'll be processing!
  
  
• Intermediate users should inspect the resulting Rejection maps and tune their Sigma High and Low to reject only artifacts like hot pixels, cosmic ray hits, satellite trails, etc. Keep everything else in order to add to your signal.
  
  
• If you are dithering (you should be!) and your data is undersampled, consider giving the new Drizzle option a try.
  
  'Screen Stretch' the master frame to see what you've got (all levels)
  
  
• Once you get your master, it doesn't look like much. It's a 32-bit image file being displayed on your 8-bit display, so all the good stuff is in the darkest pixels. Here's a great explanation by /u/EorEquis, I highly recommend it. In PixInsight, you simply hit Ctrl-A (PC) or Cmd-A (Mac) to automatically screen stretch your data. However, in this case, we get an even more red image. So we need to 'decouple' the red, green, and blue channels and re-stretch. Open the Screen Transfer Function process and click the little Chain Link button in the upper left. Then click the 'radiation symbol' button or just hit Cmd-A again to re-stretch. Voila! Your data is visible.
  
  Rename the image by double clicking its tab on the left. I usually just call the raw file 'RGB'.
  
  Background Extraction -- remove the gradients on the image from the sky glow and vignetting from equipment.
  
  
• Newbie: You can use Automatic Background Extraction to automatically model the background of the image and remove it. I set it in this case to Subtraction and it did a reasonable job with the big gradients. Since this field is rich with nebulosity, it might not always do well, but it generally only deals with big gradients, so I don't think we lost any detail.
  
  
• Intermediate: Use Dynamic Background Extraction to place your own points that represent the background. It's tough in this case because of the thick nebulosity, but it should do a fine job.
  
  
• Experienced: To better place your DBE points, try downloading a DSS frame or a well-processed image of your object from the web. You can register that image to your own data, then place your points. Then you can use those points on your real image, confident that the points match the real background.
  
  Background Neutralization and Color Calibration -- now that the big gradient is taken care of, our color channels need to match each other and reflect reality, where blue stars are blue and red stars are red.
  
  
• Newbie: Make two Preview boxes with the New Preview tool at the top. Preview 1 is a small box with the background of the image (if you can find any!). Stars can be included, but avoid nebula structure. Preview 2 is the thing you want to Color Calibrate against. The best thing for this is a face-on galaxy, but for nebula fields, I tend to pick the main DSO or even just use the entire image. Then run Background Neutralization with Preview 1 and Color Calibration with Preview 2 (white reference) and Preview 1 (background), turning Structure Detection off. When done, hit Cmd-A again to auto-stretch, this time with the Color Chain linked again in STF.
  
  
• Intermediate: Same as above, the defaults work well and you can double check the results with Histogram Transformation.
  
  
• Experienced: If you are shooting a nebula field like this one, try a new technique that I just learned. At some point in the night, take a one minute image of your field, just enough to get the colors to register. Then slew to a nearby face-on spiral galaxy at the same altitude (to account for extinction) and get the same one minute exposure. When doing Color Calibration, you can refer to this other frame and guarantee a real valid representation of white light!
  
  Histogram Transformation to 'Stretch' your data -- all along, we've kept our files 'linear', which is very good. In other programs, you immediately have to stretch your data permanently so that you can't do important steps like gradient removal and color calibration while linear. Again, I refer you to /u/EorEquis 's [excellent video explanation]((https://www.youtube.com/watch?v=lWXj6Pc_hog). We're now ready to permanently stretch the data to work on it in non-linear space, so what we see on our monitor is the real image.
  
  
• Newbie: We will simply take the automatic stretch done by STF and apply it to the Histogram Transformation process. There is a little triangle icon at the bottom left of STF, and you just drag it to the bottom bar of Histogram Transformation. This transfers the settings, so you should see the diagonal line (linear) suddenly curve tremendously -- that's the stretch! Then just apply that Histogram Transformation to the image. It turns white, meaning it's doubly stretched (by HT and STF), so hit Reset in the bottom right of the STF and it will return to looking the same.
  
  
• Intermediate: Same process, but did you know that you can tune the STF's autostretch settings? Cmd-Click or Ctrl-Click the radiation symbol button (autostretch) and you'll get a dialog of options to tune. I frequently just darken the background by lowering the Target Background, but you can also boost the highlights more if you'd like. You can of course do these things in post-processing next, but getting a nice stretch up front can save time.
  
  
• Experienced: Consider using Masked Stretch. It's not perfect for every image, but it can really help retain star cores on some data.
  
  (more to come)
  

Is she interested in using the telescope visually too? or mostly just for photography?

A "complete package" scope at that price that would be pretty decent for visual and also decent for lunar/planetary/very bright DSOs is the Celestron NexStar 4se plus a T-adaptor and T-ring for her Nikon. Total: under $500

One that would require a bit more work and luck to find used stuff, but might be worth it if she wants to continue with the hobby: Used Celestron AVX or Orion Sirius mount, some kind of power option, cheap 80mm refractor like the Orion Short Tube, T-ring for Nikon. Total: Maybe $600 if you get lucky